Remove HAVE_STRINGS_H macro.
Remove HAVE_STRINGS_H macro.

It's equivalent to OS(POSIX), and used only in StringExtras.h.  Remove it for simplicity.
This CL has no behavior changes.

BUG=

Review URL: https://codereview.chromium.org/1429883002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357307}

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